This invention relates to an optical information recording and reproducing apparatus which records information on an information recording surface by irradiating a laser beam thereon and/or reproducing the information from the information recording surface, and more particularly to an information recording and reproducing apparatus of the type suitable for use in an optical system in which simultaneous irradiation of a plurality of laser beams on an information recording surface is required or in an optical system in which laser beams of different shapes are required as necessary.
In recent years, an optical information recording and reproducing apparatus has been developed wherein a laser beam is irradiated on a disc-like record carrier with a helical track or concentric tracks having a data field and an address field so as to record information in the form of changes in optical characteristics such as surface unevenness, reflection factor or transmittivity along the track, and a change in the optical characteristics is detected to reproduce the information. This apparatus has many applications in various fields. Especially, in case where the recorded information is of digital data for use with a computer, the data is required of high reliability and must be subject to fast read and therefore, there needs a function to reproduce the information immediately after its recording for the sake of checking whether or not the information has been recorded correctly. This checking function is called a direct read during write check (DRDW), according to which a write beam and a read beam are irradiated on the same track at preceding and succeeding positions thereof so that immediately after information has been recorded with the write beam, the recorded information is reproduced with the read beam. In a prior art apparatus such as disclosed in Japanese Patent Application Laid-open No. 181441/82, two light sources of different wavelengths were used for simultaneous irradiation of the write and read beams. In this prior art apparatus using two laser light sources, however, fluxes of light from the two light sources are required to be incident on a focusing lens with one incident flux inclined with respect to the other at a very small angle in order that the write or recording beam spot can be positioned in close proximity to the read beam spot, and difficulties are encountered in achieving the fine angle adjustment. Further, temperature and vibration tend to disturb the positional relationship, thereby making it difficult to correctly record and reproduce the information.
For use in this type of optical information recording and reproducing apparatus, there have been developed a so-called write once type optical disc wherein information is recorded which takes the form of, for example, a hole locally formed by irradiating a laser beam on a recording thin film serving as the recording surface and in addition, an erasable optical disc wherein a recording film of, for example, chalcogenide material is used and the recording film is reversibly changed into a crystalline state or an amorphous state under irradiation of a laser beam to thereby provide rewritable information. In this erasable optical disc, a laser beam for recording or reproducing and a laser beam for erasing must be focused on the recording surface in different shapes, for example, a circular spot for the recording and reproducing laser beam and an elliptical spot for the erasing laser beam. Therefore, in this prior art apparatus, two light sources of different wavelengths respectively adapted for recording or reproducing and erasing are used for independent formation of the circular spot and the elliptical spot. Accordingly, this type of apparatus still faces problems that two light sources are required, that the positioning of the circular and elliptical spots is difficult to achieve, and that temperature and vibration tend to disturb the positional relationship between these spots, thereby preventing correct recording, reproducing and erasing of the information.